Achieving Superior Coatings Adhesion

The single most important requirement for most coatings is that the system adheres strongly onto the substrate after film formation has completed. Adhesion is defined as the resistance of the coating to being removed from the substrate.

Complete Wetting

Coatings are most often applied as liquid paints. For good coatings adhesion, it is crucial that the liquid fully spreads over the surface that must be coated. This pre-condition for obtaining adhesion, called complete wetting, is governed by the surface tension of the liquid and the surface energy of the solid.

Wetting is governed by surface energy and surface tension.

Complete wetting will occur when the surface tension of the liquid is lower than the surface energy of the solid. Two approaches can be used when the pre-condition of complete wetting is not obeyed:

Raise the surface energy of the solid via pre-treatment. In some cases, simply cleaning the surface with organic solvent can be sufficient as pre-treatment.

Lower the surface tension of the paint by using an additive called wetting agent(EU).

Key Aspects of Adhesion

Several aspects have to be taken into consideration when good adhesion is required. This applies to both normal adhesion, referring to a coating on a substrate, and intercoat adhesion, the adhesion of a coating onto another coating.

Adhesion and intercoat adhesion.

A variety of adhesion mechanisms can be used in order to obtain good adhesion:

The approach of assuring strong intermolecular interactions between substrate and coating is important, especially when metals are coated. Preferably strong physical bonds, like hydrogen bonds, are used.

Diffusive adhesion is possible when paint components are able to dissolve into the substrate during film formation. Diffusion can be used when plastic objects are coated.

Diffusion of binder components in plastic.

A rough surface has a higher effective surface area than a smooth surface. Surface roughness is beneficial for adhesion. It can be obtained via pre-treatment like sanding or abrasive blasting.

When a substrate is porous, like stone, liquid paint can penetrate into the pores. In many cases penetration gives strong adhesion because of mechanical anchoring.

Mechanical anchoring of a coating on a porous substrate.

Superior adhesion is obtained when the coating is chemically bonded, via covalent bonds, to the substrate. Substrate and coating are then said to be coupled. Specific additives, adhesion promoters (EU) based on silane chemistry, are used to bond coatings onto metals.

It depends upon the total system, consisting of both substrate and coating, what adhesion mechanisms can be used for obtaining good adhesion.

The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.

About Jochum Beetsma

With 25 years of experience in the coatings and ink industries, Drs. Jochum Beetsma brings expertise in physical chemistry, product development, innovation implementation and problem solving to his role as a paint and coatings industry expert at Prospector. A credentialed physical chemist and PDMA-certified New Product Development Professional, Jochum Beetsma authors in-depth technical articles for Knowledge.ULProspector.com.

Concurrently, Jochum is an independent consultant with Meritus Groep, where he has advised companies from around the world in the coatings, ink and related industries since 2004. Prior to joining Meritus Groep, he spent 13 years at DSM Coating Resins. While at DSM, he was the project manager of New Product Development projects, focusing on emulsions, as well as solvent-based, high-solids and powder coatings.

He also served as the chairman of the Project Management Platform within DSM, for which he coached project teams and prepared and facilitated complex project sessions. Previous to his time at DSM, Jochum was a coating chemist, project leader and binder coordinator for the Industrial Coatings Division of Sigma Coatings (now a part of PPG).

He has held numerous other titles within educational institutions and professional organizations. Some of his most useful experiences were gained as a facilitator and participant in brainstorm sessions during initiation of the Challenge Database, while applying the TRIZ method, and as an expert representative from industry in the Innovative Research Program (IOP).

Jochum is widely published in various international scientific magazines and has delivered lectures worldwide. He is a current lecturer and member of the Expert Team for SpecialChem. He is also a coatings technology lecturer for, amongst others, Polymer Technology Netherlands and Reed Business Education.

He currently resides in Zwolle, The Netherlands.

Contact Jochum via email (j.beetsma@meritus.nl) to learn more about his consulting services.

Dear Graham,
Thanks for your response. You are right, other chemistries are used for coupling (= covalent bonding of substrate and coating) as well, for example titanates. The stability of the silane covalent bonds that are formed is considered as being excellent. However, the storage stability of wet paints containing silane coupling agents can be critical: the type of coupling agent and the composition of the wet system have to be adjusted in order to prevent storage stability problems.
Greetings, Jochum.